The present invention relates to a vehicle steering control system for controlling a rear wheel steer angle.
Japanese Patent Provisional Publication No. 59-186773 shows one conventional example. A four wheel steering system of this example has a vehicle speed sensor, a sensor for sensing a front wheel steer angle, and a controller for controlling a rear wheel steer angle in accordance with the vehicle speed and the front wheel steer angle. In medium and high vehicle speed ranges, the controller of this example steers the rear wheels in a direction opposite to the steering direction of the front wheels for a predetermined limited time interval starting from an initiation of a front wheel steering operation, and steers the rear wheels in the same direction as the front wheel steering direction after the expiration of the predetermined time interval.
However, this conventional steering control system is arranged to control the rear wheel steer angle in accordance with only the vehicle speed and the front wheel steer angle, so that it is not possible to achieve a preset desired vehicle characteristic, and to sufficiently improve the transient steering response characteristic and the stability of the vehicle. Specifically, as to the yaw rate frequency response, it is difficult to make the yaw rate gain flat to the high frequency region, or to reduce the phase delay. Moreover, it is difficult to eliminate undesired overshoot occurring when a step steering input is applied as in lane change.